In relatively large engines, it is desirable to cool the pistons by impelling jets of lubricating oil onto their surfaces. Generally, the oil is delivered from a secondary manifold to the jets. The secondary manifold is directly connected to the main manifold, or to the line which supplies pressurized fluid thereto.
During engine startup and during low idle running, it is important that the main lubricating manifold receive as much fluid flow as possible, and at a reasonably high pressure, so that this fluid can be delivered to the bearings of the engine which rest against the crankshaft and to other bearing surfaces. However, when the prior art structure is used, wherein the secondary manifold is directly connected to the main manifold, some of the fluid flow is delivered to the jets which cool the pistons. Such cooling of pistons is, however, not necessary during startup and at low idle running of the engine, since the pistons are not being greatly heated under these relatively low load conditions. Thus, the oil delivered to the secondary manifold and therefrom to the cooling jets is basically wasted and, worse, serves to reduce the amount of oil delivered to the crankshaft bearings and to other surfaces which require lubrication.
Generally, the pump which is pumping the lubricating fluid to the manifolds has a bypass valve built directly into it or located closely downstream therefrom. Since the filters in such a system are generally downwstream from the bypass valve, a significant pressure drop can take place within the filters, particularly as they become clogged. Thus, in the typical prior art method of controlling pressure by having a bypass valve built directly into or closely adjacent the pump, the pressure actually delivered to the main manifold, and a secondary manifold when present, can be considerably lower than that set at the bypass valve. It is clear that this is undesirable, since lubricating fluid may be supplied at too low a pressure to adequately carry out lubrication of the crankshaft bearings, etc., and provide such jet action as is required to cool the pistons of relatively large engines.